Is there life elsewhere in the universe? This question has plagued mankind from time immemorial.
Early philosophers believed Earth was the centre of the universe, and that the gods essentially focused on us. The question of life elsewhere in the universe must have seemed absurd to them.
However, when Galileo confirmed once and for all that we were a small and insignificant part of the universe, the question resurfaced. Indeed, Galileo himself felt sure there was life on Mars, based on what he thought were artificial canals on its surface. But serious studies of planets in the 20th century made it clear life was not possible on other planets in our solar system. It was only in the late 20th century that we began to study the moons of other planets, and now, some harbour a suspicion that there just may be primitive life on some of the moons of these planets and in the interior of Mars. But that is still a suspicion.
However, the question has acquired a new edge since NASA sent out satellite Kepler.
The Kepler Mission, launched five years ago, has completely revolutionised the subject. From March 2009 to May 2013, it continuously observed a 100 sq degree portion (0.25%) of the sky. The mission was designed to find transiting planets for 145,000 stars by monitoring their light for three years. It was sensitive to light fluctuation of one part in 100,000. It found 1,790 host stars with a total of 2,321 planet candidates as of February 2012, and other data is still being studied. Of these, NASA has confirmed that 1750 are planets.
In just one small, fixed region of the sky, Kepler found thousands of planets around a similar number of stars. This has made us suspect that as many as 80% of all stars may harbour planets.
We believe that when stars are born, enough material is left over to form planets, and hence planets may be common in the universe. In many cases, we can see these planets transiting across the disk of the star or forcing the star to wobble just a little for us to notice. In rare cases, the gravity of the planet can actually focus and brighten the star’s light, and this too tells us a lot about planets. In fact, our studies have got so good that in many cases, we can now calculate the size and density of the planet, and in one rare case, we have even been able to estimate the cloud coverage on the planet by precisely measuring how the star light twinkles when the planet passes between the star and us. This is a staggering achievement considering the fact that the stars are typically so far away that even light, travelling at almost 300,000 km per second, takes more than a hundred years to reach us.
Amongst Kepler’s find was an Earth-like planet. Kepler 22b is the first planet roughly the size of the Earth (it is 2.4 times larger) that is within the habitable zone of a star. Its host star is slightly smaller and cooler than the Sun, and is some 600 light years from us. Its ‘year’ is 290 earth days long. If its surface is solid, it will have a very comfortable temperature of about 20° Celsius (the Earth is 30° Celsius). On the basis of this, scientists estimate that around one in five Sun-like stars have an “Earth-sized” planet in the habitable zone, so the nearest would be expected to be within 12 light-years distance from Earth.
So, are we about to find life in the universe? Here, the question gets difficult. Remember, even in our own solar system only one planet in eight can sustain life. But as a result of all these studies, scientists have focused a lot of energy on how life can arise in the universe. And the more we understand that, the more we realise life is a rare gift. Several conditions have to be met by a planet and its parent star before it can be gifted with life.
The first question that arises is: what is life? We’d probably find it easier to define life by what it is not. A dictionary definition states that living objects are a region of order which use energy to maintain their organization against the disruptive force of entropy. Clearly, it is highly accurate but not very helpful. Nominally, we assume life consumes food, reacts to its environment in a complex manner, grows and self-replicates, and is accompanied by a large number of chemical reactions.
The most important feature of life is that it has molecules that know what the living object looks like, and also knows how to make a copy of itself. This in itself implies that living molecules will be large and complex. The only element that we know is abundant and can make such large molecules is carbon. It seems that wherever life is found, it will be carbon based. This is because carbon is capable of accepting all types of atoms (both acidic and basic) in reactions, capable of forming really long chains of chemicals, can make very flexible chains with reversible reactions, can form both, close chained and open chained molecules which are very versatile, and is easily available in the universe.
To make matters simpler, carbon has a companion in the water molecule, a unique substance that has wide temperature range where it remains liquid, is an excellent solvent, is chemically a simple molecule that consists of equally abundant elements, and has high specific heat and high vapour pressure which allows it to store heat. However, such a life form need not be oxygen-breathing or biped.
In astronomy, we find many sites which have carbon or organic molecules in clouds and other cool regions. However, we have never seen molecules with more than a few atoms – except for a few carbon nanostructures – while even the simplest molecules of life have a few hundred thousand atoms. Also, molecules like the DNA, which are life’s instruction manuals, are like libraries. They are completely useless unless one knows how to read the instructions. Hence, living molecules require an entire scaffolding of supporting molecules simultaneously at one place. This seems to be very difficult (but clearly not impossible – after all, we do exist).
The conditions needed for evolution are not difficult to define. When the universe was born, it consisted almost entirely of hydrogen. All the other elements we see around us were created in the cores of stars. As the stars died, they dispersed these elements in the neighbouring regions. Hence, if all the life-giving atoms and molecules are to be made, they will exist in residual material around small, second or third generation stars. These materials will then form the planets that support life around other stars. Such planets will need a very coherent environment where the temperature differences and environment are stable. The star itself will have to be stable over a long period of time. How common it is, is a matter of opinion and debate. Some even argue that planets are not needed for this, but the conventional wisdom is that planets are necessary to precipitate life.
So how did life arise in the universe? Various unsatisfactory ideas have been suggested.
The most common of these is that it arose by chance. But the probability that all molecules of life arose suddenly is very small. It seems more probable that there was a certain build-up of increasingly complex molecules of all kinds from where molecules that support life replicated themselves more efficiently compared to random molecules.
It seems likely that all organic molecules (molecules that contain carbon) have properties that we do not yet know of which seem to allow them to organise themselves into self-replicating molecules and their companion molecules. It is even suggested that there may be some water soluble minerals that stick to rocks and could have helped facilitate the accumulation of such molecules.
Two scientists, Stanley Miller and Harold Urey at the University of Chicago, conducted a famous experiment in 1953, where they put soil, water and other ingredients that must have existed in the early Earth’s environment, and passed electric current through them as would be produced by lightening. They found that such process spontaneously created a lot of organic molecules, many of which are useful amino acids that help life. So, it is quite likely that through a cumulative effect of several million ‘Miller-Urey’ experiments, the Earth accumulated life-supporting molecules.
Considering that all living molecules on earth have a left-handed double helix of four nucleobases, it seems that a single successful accumulation was enough to trigger the evolution of life on Earth. Even then the details of this acceleration are difficult to fill in and there have also been suggestions that life on Earth arose in some exotic, more favourable environment elsewhere in the universe – that life on Earth was seeded from outside.
Any planet that supports life must exist in what is called ‘Habitable zone’ around a star. This is defined as the distance from its parent star so that liquid water and other elements can exist in an active form. For example, if carbon and water are frozen (as ice) the planet cannot sustain life. It should have liquid water. It cannot get so hot as to photo-dissociate water, and this implies that, for our sun, the planet should be at a distance greater than 0.95 AU (1 AU, or astronomical unit, is the average distance between the Sun and the Earth).
The outer edge is defined by the CO2 (carbon dioxide) condensation temperature, which will make carbon unavailable for reactions. This gives a maximum distance of 1.37 AU for a liveable planet around the Sun. A planet within this region can, in principle, support life. However, there are other requirements too. The planet must have a greater abundance of carbon, sodium, magnesium, and silicon for an inner solar system’s long-term habitability because the abundance of these elements make the star cooler and cause it to evolve more slowly. The abundance of oxygen in the star tends to make it more stable, in particular, and seems crucial in determining how long newly formed planets stay in the habitable zone around their host star. If our own sun had a lower abundance of oxygen, for example, Earth would have left the habitable zone a billion years ago, well before complex organisms evolved.
All this allows us to narrow down where to look for a star around which a liveable planet may support life. But even within this habitable zone, the planet must have a correct mixture of internal heat and stellar radiation so that it can recycle its internal material through volcanoes, earthquakes, etc. even as it uses sunlight to sustain its surface temperature.
Some of these conditions are even met by some of the moons of Jupiter and Saturn because they lie in the region beyond the ‘ice line’ (where the heat from the star is not intense enough to liquefy water) of the solar system and have a rocky core. Their top layers are protected from the vagaries of space by a layer of ice. They seem to harbour a liquid ocean, probably of water. They seem to be geologically active. The heat and light coming from the host planet can provide an energy source. Recently, water plumes have been seen on Jupiter’s moon Europa. Water jets have also been seen on Enceladus, a moon of Saturn. Water is known to have once flown on Mars. This suggests liquid water may be more common than we thought. However, even if they harbour life, it will be primitive since advancing from simple to complex life forms requires far greater amounts of all resources.
In spite of these severe limitations, a list of about 43 potentially habitable planets is now available on Wikipedia.
However, for intelligent life to evolve, the conditions will need to be even more restrained. The planet will need a correct temperature range, correct size and structure, a moon to stabilize its orbit, outer planets to protect it, and a long lived stable and isolated star so that it is not nudged away by the gravitational pull of a nearby star. The planet will have to be in nearly circular orbit and its axis of rotation should be so tilted that it can have seasons and yet the temperature fluctuations are not large. It will need a magnetic field to protect its surface from radiation from stars.
Life on a planet in the habitable zone ranging from simple self-replicating molecules to intelligent life will need to transform from molecule to a cell, then from a cell to a multi-cellular being. It then has to discover that sexual reproduction improves survival chances. Importantly, it will need to evolve to a stage where intelligence is a good survival strategy. Remember, as long as dinosaurs roamed the Earth, mammals had no chance of evolving into an active life form. Beyond this, technological advances far beyond survival needs will have to be acquired for intelligent life to evolve.
Even if intelligent life exists in the universe, contacting them will not be easy because of the vast distances. Travelling will certainly not be easy as long as we cannot find a way around relativity, which states that one cannot travel at velocities close to the velocity of light, let alone exceed it. So the fastest rockets will take several decades to even reach the nearest star. Assuming other intelligent life can get around relativity (or are willing to travel for thousands of years), they will have to be technologically sophisticated. We must hope they have overcome the desire to dominate, if not conquer, or we are in serious trouble.
In fact, some scientists have even argued that the very fact that we have not had ‘visits’, super-intelligent life capable of interplanetary/interstellar travel doesn’t exist. I may add that there have been no reliable and verifiable records of unidentified flying objects (UFOs) which we can take seriously.
Dr Mayank Vahia is a scientist working at the Tata Institute of Fundamental Research since 1979. His main fields of interest are high-energy astrophysics, mainly Cosmic Rays, X-rays and Gamma Rays. He is currently looking at the area of archeo-astronomy and learning about the way our ancestors saw the stars, and thereby developed intellectually. He has, in particular, been working on the Indus Valley Civilisation and taking a deeper look at their script.